build: cache compiled dependencies in image builds

.github/workflows/build.yaml

@@ -142,10 +142,18 @@ jobs:
     steps:
       - uses: actions/checkout@v7
 
+      # The gha cache backend needs a buildx builder (the default docker
+      # driver cannot export a cache).
+      - uses: docker/setup-buildx-action@v4
+
       - uses: docker/build-push-action@v7
         with:
           context: .
           # The Dockerfile lives under docker/; the context stays the repo
           # root so COPY paths are unchanged
           file: docker/Dockerfile
+          # Every layer, the compiled dependencies included, kept in the
+          # Actions cache: builds on main fill it, every build reads it.
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
           push: false

.github/workflows/release.yml

@@ -105,10 +105,18 @@ jobs:
             type=sha,prefix=
             type=raw,value=latest
 
+      # The gha cache backend needs a buildx builder (the default docker
+      # driver cannot export a cache).
+      - uses: docker/setup-buildx-action@v4
+
       - uses: docker/build-push-action@v7
         with:
           context: .
           file: docker/Dockerfile
+          # Every layer, the compiled dependencies included, kept in the
+          # Actions cache: builds on main fill it, every build reads it.
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
           push: true
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}

docker/Dockerfile

@@ -8,23 +8,32 @@ RUN apk add --no-cache git \
 WORKDIR /ui
 RUN npm ci --no-audit --no-fund && npm run build:gitops
 
-# Stage 2: build
-FROM rust:1.96-slim-trixie AS builder
-
-RUN apt-get update && apt-get install -y pkg-config libssl-dev curl && rm -rf /var/lib/apt/lists/*
-
-WORKDIR /app
-COPY Cargo.toml Cargo.lock ./
+# Stage 2: build, in two steps so a release only compiles what changed.
+# cargo-chef reduces the manifests to a recipe (the service's own version
+# masked, so release-please's bump does not count), and `cook` compiles the
+# dependencies from it alone: that layer is reused, from the CI build cache,
+# for as long as Cargo.lock's dependencies stay the same. Any change to them
+# rebuilds the whole layer once.
+#
 # --locked: build exactly the versions CI tested. Without it, cargo may
 # quietly re-resolve inside the image and ship dependency versions the test
 # suite never saw.
-#
-RUN mkdir src \
-    && echo 'fn main() {}' > src/main.rs \
-    && cargo build --release --locked && rm -rf src
+FROM rust:1.96-slim-trixie AS chef
+RUN apt-get update && apt-get install -y pkg-config libssl-dev curl && rm -rf /var/lib/apt/lists/*
+RUN cargo install cargo-chef --version 0.1.78 --locked
+WORKDIR /app
+
+FROM chef AS planner
+COPY Cargo.toml Cargo.lock ./
+COPY src/ src/
+RUN cargo chef prepare --recipe-path recipe.json
 
+FROM chef AS builder
+COPY --from=planner /app/recipe.json recipe.json
+RUN cargo chef cook --release --locked --recipe-path recipe.json
+COPY Cargo.toml Cargo.lock ./
 COPY src/ src/
-RUN touch src/main.rs && cargo build --release --locked
+RUN cargo build --release --locked
 
 # Stage 3: runtime
 FROM debian:trixie-slim